The sail of a boat at an angle of incidence to the wind develops low pressure along its leeward side and high pressure along its windward side by reason of the airfoil shape of the sail. This in turn establishes circulation of air about the sail, or a bound vortex, that generates lift and drag forces, which also can be resolved into a thrust component that moves the boat forward and a heel component directed perpendicular to the boat. Equal and opposite hydrodynamic forces are generated by the underwater foils of the boat, i.e., keel or centerboard and rudder, acting at an angle of incidence, or leeway angle, to the flow of water about their surfaces. With an increase in windspeed, several adverse regimes develop wherein the heeling force increases at a greater rate than the forward thrust component of the increased sail force. The first is in the range of moderate to fresh winds, such as windspeeds in the range of about 10 to 20 knots, for example, when the air flow about the leeward side of the sail becomes increasingly turbulent and the sail produces a reduced thrust component but an increased heel component. The second adverse regime is in the range of strong winds and above, such as windspeeds of about 20 knots and higher, in which the heel component increases still further so as to cause an excessive amount of heel whereby the underwater foils work less efficiently and boat slips sideways. In both regimes, the lift developed by the sail increases with the increasing wind strength, but the drag component increases at a faster rate, the resulting effect of which is that the forward thrust component is reduced.
The prior art does not have an effective way of handling the deleterious effect of the turbulent air flow of the first regime in a manner that will maximize boat performance; the sailor, therefore, is not able to realize the full potential of the increased lift, i.e. obtaining a greater measure of forward thrust with its attendant enhancement of boatspeed. The prior art, however, has several techniques of dealing with the strong wind conditions of the second regime set out above, all of which involve shortening sail area. In the case of the typical sloop rigged sailboat, these techniques include reefing the mainsail or changing or furling the jib. The mainsail is most often reefed by a slab reefing system that results in shortening its luff length to reduce sail area; another system involves furling the mainsail inside the mast, but this is in limited use at the present time. With respect to headsails, the technique most often used by racing boats is to change to successively smaller jibs as the windspeed increases. Many cruising sailboats now use a roller furling jib system by which a jib is rolled about a headstay having swivels at its top and bottom ends which enables a jib to be rolled into a progressively smaller size as the wind increases.
The foregoing sail shortening techniques have various disadvantages. Reefing the mainsail results in a poor airfoil shape as compared to the shape built into the full sized main. Slab reefing requires several people to execute the method, and the boat must be sailed in an inefficient condition during the maneuver. Roller furling the main inside the mast also results in an inefficient airfoil shape and has the additional disadvantage of requiring a large mast and associated equipment that results in substantial expense. The jib roller reefing systems also degrade the sail shape as the jib is rolled about the headstay, and the boat is limited to using one jib as it is impractical to change a jib that is rigged for roller furling. Although jib roller furling gear is of moderate expense, it is subject to breakdown or malfunction. Also, a jib must be specially constructed for roller furling and a special sail must be obtained if the system is to be retrofitted to a boat. All of these sail shortening techniques cause extra wear of the sail. Changing to smaller jibs requires several people to take one sail off and hoist another, and skilled people are needed to perform the maneuver in view of the risk involved when working on the bow of a boat in strong wind and rough sea conditions, which is further exacerbated when changing jibs at night.
One of the principal objects of my present invention is to provide a new sail construction that obviates the disadvantages of the prior art sail shortening systems described above. Another principal objective of my invention is to provide a sail construction that also improves the flow of air about a sail in moderate to fresh wind conditions. Still further main objectives are to provide a sail construction with the foregoing capabilities that can be built into a new sail, and also to provide an embodiment thereof that can be retrofitted to an existing sail. These and other advantages and objectives of the invention will become apparent and explained in the detailed description which follows.